1. Technical Field of the Invention
The present invention relates to a key module used with key-actuated membrane switch panels including a flat spring which is unilaterally stretched within the key housing, as well as to a method for adjusting switch-over time.
2. Discussion of Relevant Information
A key module for membrane switch panels is disclosed by British Patent GB-A No. 2,141,874, in which the key housing has a bottom side on which a flat spring, bent at a right angle, extends unilaterally. The bent end of the flat spring is inserted from the bottom of the housing into an opening in the key housing. A plunger is displaceable within the key housing, and the plunger acts on the free end of the flat spring to bend the flat spring downwardly, whereby an upper membrane of a membrane switch array is bent through a free area in a spacer against a lower membrane in order to place the contact point on the membrane into contact therewith. The plunger is guided within a tubular portion of the housing which, with respect to the central point of the mounting surface of the key module, is located oppositely from the slot, through which the stretched end of the flat spring can be taken out. A key cap can be attached to the top of the plunger. Insofar as the flat spring does not have adequate rigidity in order to achieve desired spring characteristics with a specified operating force, a compression spring is additionally provided and is arranged between the key cap and the key housing, the compression spring being positioned about the tubular portion which incorporates the plunger. This arrangement is disadvantageous insofar as the key module is capable of functioning only in a completely assembled condition, and, therefore, during production it is not possible to check the spring characteristics and the state of the switch-over time.
As discussed in European Application EP-A-187,396, a key module is provided which does not use such an additional compression spring; however, this key module requires a mounting space which extends over the modular area of the two key modules.
The key cap in British Pat. No. 2,141,874 is attached only to the plunger, and, further, the distance of the bottom side of the key cap from the key housing determines the span of the additional compression spring. As a result, it is difficult (in view of the unavoidable use of large mounting tolerances which are necessary to secure the plurality of key modules which must be accommodated within a single keyboard) to provide a constant initial operating force having very narrow tolerances. Therefore, it is desirable to provide a plunger stroke termination point with which the initial value of the operating force of the additional compression spring can be determined precisely during assembly of the key modules.
However, for a preferably designed keyboard, it is not only desirable that single key modules can be operated with precisely the same initial operating force, but also that switchover time, i.e., the time during which switching occurs, will be adjusted as precisely and as definitely as possible. During the process of inserting a flat spring into a key module in accordance with British Pat. No. 2,141,874, a large spread of different switch-over times is unavoidable, insofar as repositioning of the flat spring is very difficult to adjust. Further, not only the positioning of the flat spring but also, to a certain degree, the rigidity of the upper membrane, must be determined for a timely spreading of the switch-over time. It follows that it is desirable to be able to adjust the switch-over time not only during assembly of the key module, but also after assembly of the key module on a membrane switch panel, particularly for high priced keyboards, such as keyboards for word and data processing systems.
Therefore, the invention has an object the provision of a key module for a membrane switch keyboard, with the switch-over time being adjusted as precisely as possible, specifically with respect to the switch-over time as well as to the operating force required during the switch-over time.
The problem is solved by a key module formed in accordance with the claims and a method for adjusting switch-over time as defined by the claims.
Additional details of the invention are recited in the depending claims.